If I make a gallery post showing whole picture with a different ratio of my browser window I can see the background picture both side of the main gallery picture. How make the background picture darker or black and see just the main picture??

After a try, I can see that it make the background darker all the time (on all kind of post) but, if it’s possible, I would like make the background darker only with the gallery post type like it is when I open a prettyphoto gallery in a text type post.